The present invention relates generally to tank gaging systems, and more particularly to a system for anchoring gaging apparatus to the bottom of fluid storage tanks. In addition, the present invention relates to a device and method to be used to calibrate tank gaging systems.
The use of fluid level measuring systems in fluid storage tanks is well known in the industry. Tank gaging systems typically employ a float which floats at the air/liquid interface within a tank. The positions of the float are calibrated to indicate how much liquid is in the tank at each given float position. Thus, a known quantity of liquid may be added to the tank, and the positions of the float relative to the bottom of the tank noted before and after tile addition. This procedure may be repeated to obtain a complete calibration of the tank and its gaging system. As is more frequently the case, however, one or more such observations may be used in conjunction with the known specifications of the tank to arrive at a complete calibration.
The fluid level measuring systems used in gaging tanks usually include pipe or tubing made of synthetic resins such as polyvinyl chloride, stainless steel, or other non-ferrous metals, which extend down into a tank from the top of the tank. The top of the pipe is connected to the top of the tank with a flange. In the past, the lower end of the pipe has been left to hang free, has been bolted to the bottom of the tank in some fashion, or even held down with heavy weights. A float is coupled to the pipe so as to move vertically along the pipe.
One specific gaging system marketed by the Bindicator Co., Port Huron, Mich., employs a float containing a magnetic member which triggers magnetic sensors positioned within and along a vertically disposed pipe.
When a fluid level measuring device is left to hang free in a tank it is very susceptible to being whipped around by fluid forcing its way into the tank. This whipping action, in turn, often causes the pipe portion of the device to be damaged. When the pipe breaks or cracks, the fluid may enter the sensitive measuring device, damaging it and causing it to have to be replaced. In severe instances, the lower part of the fluid measuring device may actually break off and fall to the bottom of the tank. This requires the tank to be drained of fluid so that someone can enter it and retrieve the broken parts.
The method of bolting a fluid level measuring device to the bottom of a tank is costly, time consuming, and above all dangerous. To bolt the fluid level measuring device to the bottom of a fluid storage tank requires that a person enter the hostile tank environment. To do this, the storage tank must first be emptied of fluid--a time consuming process which requires a costly vacuum truck to be on location. Next, sludge that has collected on the bottom of the tank must be cleaned out. To do this, a person must enter the tank to steam or wash it clean. At this point, safety is of the utmost concern because a tank may contain many dangers. For instance, in oil field or sewage treatment tanks, flammable hydrocarbon gas may well be present. In addition, poisonous gasses such as H.sub.2 S may also be present. In general, therefore, a tank has to be vented thoroughly before anyone can enter the tank. This normally requires either a compressor on location to do the job quickly, or plenty of time to allow the tank to vent naturally. More than likely, however, time will be of the essence because the tank will typically be needed to store incoming fluid. Once a tank has been vented, it can then be cleaned so that it will be possible to work inside it. Once the tank has been cleaned, a person can then enter and bolt the fluid level measuring device to the bottom of the tank. The tank is then resealed and placed back in service.
The method of using a heavy weight to hold down a fluid level measuring device to the bottom of a tank is also fraught with problems. Thus, the tank bottom still needs to be cleaned at least partially, so that the weight can sit on the bottom of the tank. Further, to get a weight that will hold the fluid level measuring device steady in strong flowing fluids, the weight will have to be both heavy and bulky. And to install the weight without emptying a tank, the weight will have to be carried to the top of the tank which will often be 20 feet high or more. Even after taking all of these precautions, a weight that is seated on a slant- or cone-bottomed tank may slide down the bottom of the tank, thereby causing the fluid level measuring device to move resulting in errors in the calibration of that device.
Once a fluid level measuring device has been installed by any method including those discussed above, the device must be calibrated to give a correct reading of liquid level. In that regard, it is well known in the art to manually measure liquid level with a plum-bobbed measuring tape. Typically, the value read on the tape is the height at which the float of the fluid level measuring device sits on the liquid. This value is entered as a set point for calibrating the gaging device. After reading one value, the liquid level in the tank is either raised or lowered by a given volume of liquid, and a new liquid level is noted on the tape. This second value is also entered as a calibration set point. By thus entering these set points, the fluid level measuring device can be calibrated to give accurate readings of fluid level at all float locations. It will be apparent, however, that the process just described is both time consuming and difficult, mainly because it requires raising and lowering the fluid level in a large fluid storage tank. What is needed is a method to calibrate the fluid level measuring device without having to raise or lower the fluid level in the tank.